1. Field of the Invention
The present invention relates to the polymerization of lactide (either S or R) using a bismuth, scandium, yttrium or lanthanide series rare earth metal based catalyst.
2. Description of the Related Art
Lactides are presently polymerized to high molecular weight plastics using tin, titanium, zinc and other metal based catalysts by ring opening polymerization of the cyclic ester: ##STR1## The resulting polymers of lactide are useful in medical applications such as wound closure devices, orthopedic implants, controlled release drug carriers, as well as degradable paper coatings and plastic packaging films.
A. J. Nijenhuis et. al., disclosed lactide could be polymerized using Sn(acetylacetonate).sub.2 and substituted Zn(II) acetylacetonate complexes as catalyst. They claimed the very high crystallinity polymer was produced because the polymerization rate was lower than the crystallization rate, thus allowing polymeriation directly onto the p-lactide crystal (Polymer Bull. (1991), 25, pp. 71-77).
U.S. Pat. No. 4,853,459 discloses ring-opening polymerization of cyclic carbonates at 200.degree.-350.degree. C. using coordination compounds of lanthanides such as Cerium tris(acetylacetonate).
Shen, Z.; Sun, J. and Wu, L., Huaxue Xuebao, 48(7), pp. 686-689(1990), disclose solution polymerization of DL-lactide using a mixture of aluminum alkyls, rare earth compounds, and water.
U.S. Pat. No. 5,028,667 discloses the polymerization of various lactones including lactide using yttrium and lanthanide series rare earth based catalysts. In a commonly assigned U.S. application serial number CH-1883, concurrently filed herewith, preferred catalysts within U.S. Pat. No. 5,028,667 for use in melt polymerizations of lactide are disclosed.